Over a 25-month period, six multiply transfused patients undergoing cytotoxic treatment for haematological or other malignant disorders developed icteric acute hepatitis B virus (HBV) infection. Bone marrow or peripheral-blood stem cells had been harvested from all six patients and stored in the same cryopreservation tank for possible future transplantation. Human DNA, HBsAg, and HBV DNA with sequences identical to those from four patients with related infections were subsequently found in the liquid nitrogen. Leakage of the cryopreservation bags used to store bone marrow harvested from the first patient when acutely infected with HBV led to contamination of the tank and its contents with HBV and subsequent transmission to patients after transplantation. This incident emphasises the continuing need to screen donors of tissue to be cryopreserved for bloodborne virus infections. It also reinforces the requirement for primary containers used to cryopreserve human tissue to be sealed in a way which prevents exchange of material between the specimen and the liquid nitrogen.